• Photolithotrophy;
  • chemo-organotrophy;
  • ammonium nutrition;
  • nitrate nutrition;
  • nitrogen fixation


Iron has many catalytic roles in photosynthesis, respiration and nitrogen assimilation; molybdenum is involved in NO3 and (usually) N2 reduction. This paper concentrates on computing the efficiency of use of Fe and Mo in growth of organisms growing with different mixtures of the major resources photons, carbon and nitrogen. This computed efficiency is defined as mol C assimilated (mol catalytic metal in the organism)-1 s-1. The efficiency is computed from the known in vivo involvement of Fe- and Mo-containing enzymes, the specific reaction rate of these enzymes, the specific rate of these enzymes in vitro, and the growth requirements for the products of the reactions, for organisms using different energy, carbon and nitrogen sources. The predicted Fe use efficiency is greater for chemo-organotrophic than for photolithotrophic growth with a given nitrogen source, and increases in the order N2 NO3- NH4+ for growth with different nitrogen sources in the presence of constant photon and carbon availability. The predicted changes with variations in nitrogen source are greater than those with variations in photon and carbon source. These predicted values may be compared with observed metal use efficiencies, defined as mol C assimilated (mol total metal in the organism)-1 s-1. Relatively few such observed values are available From the literature. For Fe in photolithotrophically growing microalgae the observed values can be as low as 16–17% of the predicted values, i.e. 83–84% of the Fe in the organism is not accounted for by known catalytic uses of Fe if the catalysts are operating at their maximum specific reaction rates. One possibly discordant prediction relates to the Fe and Mo cost of N2 fixation; the computations suggest that in vitro estimates of nitrogenase activity may be less than those sometimes found in vivo. The predictions also have possible implications for resource availability interactions (photons, carbon, nitrogen, Fe, Mo) in natural conditions.